Because their citizens tend to be more “swarthy”, and in some cases “non-Christian”?
“There were no red-haired Irishmen, until the Vikings came to Ireland
And how many Romans had dark curly hair before they brought slaves from Africa?
No race on Earth is completely pure,
Nor is anyone’s mind and that’s for sure.
The winds mix the dust up from every land
And so will woman and man.”
(not sure who wrote it, I heard from Pete Seeger.)
BTW-- why is it on Star Trek everyone still seems to be clearly ethnically defined? You’d think 300 years into the future (and after a devastating world war) things would’ve gotten a lot more mixed up. Instead we get humans who all seem to be from one nation or the other, but occasional mixes between humans and Klingons or Vulcans.